This invention relates to a process for producing an olefinic copolymer rubber having good processability, a high tensile strength and a highly random molecular arrangement in good yields using a solvent-soluble Ziegler-type catalyst.
Catalyst systems composed of homogeneous vanadium compounds and organoaluminum compounds have been frequently used for production of rubbery copolymers by random copolymerization of at least two olefins. In general, however, these homogeneous vanadium catalysts are very liable to undergo deactivation during polymerization and do not have high activity at a temperature of 30.degree. to 60.degree. C. which is a practical polymerization temperature.
On the other hand, catalyst systems composed of titanium compounds and organoaluminum compounds are generally known to undergo less deactivation in polymerization. Use of these titanium compounds, however, tends to result in homopolymerization of the olefins to give a mixture of homopolymers. Or even if copolymerization does occur, block copolymers tend to form. Accordingly, the production of rubbery copolymers by titanium-type catalysts has not been commercially successful.
Some patent documents were recently published which are directed to the production of ethylene/alpha-olefin copolymer rubbers with titanium-type catalysts (for example, Japanese Laid-Open Patent Publications Nos. 51381/74, 117886/75 and 104687/78). The methods disclosed in these patent documents involve random copolymerization of ethylene with an alpha-olefin using a catalyst composed of (i) a solid titanium catalyst component consisting of a titanium compound supported on a carrier and (ii) an organoaluminum catalyst component. Since the resulting copolymer does not have a highly random molecular arrangement, it deposits partly on the hydrocarbon solvent used in the polymerization. It is difficult therefore to obtain rubbery olefinic copolymers by solution polymerization using the catalysts suggested in these patent documents. Moreover, the resulting copolymers are plastics, and are difficult to use in the field of rubbers.
The present inventors therefore worked on a process which involves using a titanium compound in solution instead of a solid titanium-type catalyst, and found that when at least two olefins are copolymerized using a catalyst composed of (a) a titanium compound being soluble in an organic solvent and having the ability to catalyze polymerization of olefins, such as titanium tetrachloride, titanium triacetylacetonate or tetrabutoxy titanium and (b) an organoaluminum compound, a mixture of homopolymers of the olefins or a block copolymer of the olefins tends to form and a rubbery copolymer of the olefins cannot be obtained.
The present inventors further worked on a process which involves using a catalyst a liquid titanium compound obtained by reducing titanium tetrachloride while maintaining it liquid.
A method for preparing such a liquid titanium compound is known, for example, from Japanese Laid-Open Patent Publications Nos. 16298/76, 76196/76 and 148490/77 which specifically disclose a method comprising reducing titanium tetrachloride with an organoaluminum compound in a hydrocarbon solvent in the presence of an ether. Japanese Laid-Open Patent Publications Nos. 78691/77, 152485/77 and 153896/77 disclose a method which comprises reducing titanium tetrachloride with an organomagnesium compound in an aromatic hydrocarbon solvent in the presence of an ether. In the methods disclosed in these patent documents, the liquid reduction product of titanium tetrachloride so formed is heated, or a liberating agent is added to it to precipitate titanium trichloride which is then separated from the ether to use it as a catalyst for stereospecific polymerization of propylene. In the liquid reduction product of titanium tetrachloride, the ether is present in an amount of at least 1 mole per mole of titanium, and it has been known heretofore that when titanium is present together with at least one mole of an electron donor, its activity as a catalyst for polymerization of olefins is very low or does not exist at all.